The present invention envisions a threaded fastener and a blank used to form the threaded fastener which provides improved thread forming characteristics.
A variety of thread forming fasteners are available which form threads in an unthreaded pilot bore. An example of a well known self-tapping threaded fastener is the tri-lobular family of fasteners. This family of fasteners uses a fastener blank having a generally triangular cross-sectional profile. The cross-sectional profile is not truly triangular but rather has the form of arcuate surfaces which are positioned in a triangular orientation. For example, three generally arcuate surfaces are formed on a shank and are connected by smaller radius arcuate surfaces. All of these arcuate surfaces are generally axially oriented along the shank of the fastener blank. The fastener blank is then processed through a roll die to form threads thereon. The thread forming process results in three thread forming lobes protruding from the shank.
The foregoing example of the tri-lobular family of fasteners is but one example of various cross-sectional geometries used in the thread forming fastener art. The general principle is to provide a cross-sectional profile geometry which produces a plurality of protruding lobes for forming threads in a pilot bore. While a variety of fasteners have been devised, they do not necessarily optimize various desirable thread forming characteristics. For example, while the tri-lobular fastener structure provides sufficient thread forming characteristics, it may not optimize the pull-out or strip-out characteristics for various applications. Also, some embodiments of prior art thread forming fasteners may tend to wobble when initially driving the thread forming fastener into the unthreaded pilot hole. It would be desirable to prevent this wobbling so as to provide a more efficient and reliable thread forming fastener and formed thread within the bore.